Optimize branch commit resolution

app/graphql/resolvers/branch_commit_resolver.rb

@@ -7,12 +7,16 @@ module Resolvers
     alias_method :branch, :object
 
     def resolve(**args)
-      return unless branch
+      commit = branch&.dereferenced_target
+      return unless commit
 
-      commit = branch.dereferenced_target
-      project = Project.find_by_full_path(commit.repository.gl_project_path)
+      lazy_project = BatchLoader::GraphQL.for(commit.repository.gl_project_path).batch do |paths, loader|
+        paths.each { |path| loader.call(path, Project.find_by_full_path(path)) }
+      end
 
-      ::Commit.new(commit, project) if commit
+      ::Gitlab::Graphql::Lazy.with_value(lazy_project) do |project|
+        ::Commit.new(commit, project) if project
+      end
     end
   end
 end

spec/graphql/resolvers/branch_commit_resolver_spec.rb

@@ -12,11 +12,11 @@ RSpec.describe Resolvers::BranchCommitResolver do
 
   describe '#resolve' do
     it 'resolves commit' do
-      is_expected.to eq(repository.commits('master', limit: 1).last)
+      expect(sync(commit)).to eq(repository.commits('master', limit: 1).last)
     end
 
     it 'sets project container' do
-      expect(commit.container).to eq(repository.project)
+      expect(sync(commit).container).to eq(repository.project)
     end
 
     context 'when branch does not exist' do
@@ -26,5 +26,19 @@ RSpec.describe Resolvers::BranchCommitResolver do
         is_expected.to be_nil
       end
     end
+
+    it 'is N+1 safe' do
+      commit_a = repository.commits('master', limit: 1).last
+      commit_b = repository.commits('spooky-stuff', limit: 1).last
+
+      commits = batch_sync(max_queries: 1) do
+        [
+          resolve(described_class, obj: branch),
+          resolve(described_class, obj: repository.find_branch('spooky-stuff'))
+        ]
+      end
+
+      expect(commits).to contain_exactly(commit_a, commit_b)
+    end
   end
 end